The etiology and pathogenesis of most types of inflammatory arthritis remain speculative. The role of the adaptive immune system in inflammatory arthritis has been extensively studied. Recently, the innate immune system has been recognized as a possible critical component to initiating or sustaining chronic inflammatory arthritis. The innate immune system is activated by pattern recognition receptors, including NOD- like receptors (NLR) and Toll-like receptors (TLR). This application focuses on one NLR, nucleotide oligomerization domain-2 (NOD2). NOD2 is an intracellular molecule that detects muramyl dipeptide (MDP), a breakdown product of bacterial cell walls, and activates host defenses. NOD2 also plays a critical regulatory role in the control of immune homeostasis. This is known because mutations in NOD2 cause Blau syndrome (a disease characterized by inflammatory arthritis, and inflammation in the skin and uveal track of the eye), and are associated with Crohn's disease, atopic disorders (allergy, asthma and eczema), and possibly colorectal cancer. NOD2 is also expressed in atheroma lesions of patients with coronary artery disease. During the current period of Merit Review funding the principal investigator (PI) identified a novel protein-protein interaction between NOD2 and OAS2, a protein involved in the innate immune response to viruses. This proposal describes studies to follow-up on this observation. Also during the current funding period, using a model of arthritis triggered by an adaptive immune response to proteoglycan aggrecan, the PI found arthritis was worsened when NOD2 was activated by MDP. Studies in this proposal will continue to explore the role of NOD2 in the control of immune homeostasis and inflammatory arthritis. The following experiments are proposed. 1) The nature of the interaction between NOD2-OAS2 will be investigated using biochemical approaches. The ability of each protein to influence the activity of the other in innate immune functions will be investigated. 2) Using normal and NOD2 knockout mice, analyze inflammatory arthritis triggered by intra-articular injection of peptidoglycan from cell walls of Gram positive bacteria, b) zymosan and c) double stranded viral RNA and d) by systemic administration of live Group B streptococci. The role of NOD2 in influencing several parameters of inflammation will be evaluated. The course of arthritis when NOD2 is intentionally activated by intra-articular MDP will be characterized as will the specificity of suppression following NOD2 activation by systemic administration of MDP. Potential Impact on Veterans Health Care: Chronic inflammatory conditions causing arthritis, uveitis, dermatitis, inflammatory bowel disease, asthma and coronary artery lesions represent a significant health issue for veterans. Genetic studies have shown that NOD2 is critical to controlling chronic inflammation in multiple organs. Understanding how NOD2 controls the development of inflammation leading to tissue injury and pathology will have broad implications for the health of veterans. PUBLIC HEALTH RELEVANCE: Chronic inflammatory conditions causing arthritis, uveitis, dermatitis, inflammatory bowel disease, asthma and coronary artery lesions represent a significant health issue for veterans. Genetic studies have shown that NOD2 is critical to controlling chronic inflammation in multiple organs. Understanding how NOD2 controls the development of inflammation leading to tissue injury and pathology will have broad implications for the health of veterans.